Carbon nanotubes offer excellent mechanical and electrical properties. For this reason, they are expected to find applications in a wide range of fields such as electron source materials and conductive resin materials. In particular, there is a strong demand for applications that utilize the excellent electron characteristics of single-walled carbon nanotubes (hereinafter referred to as “SWNTs” in this Specification). However, practical applications have largely been hindered by their poor solubility and dispersibility (owing to the fact that carbon nanotubes are aggregated in a bundled state).
In the technical fields in question, dispersing bundled SWNTs in an organic solvent as individual nanotubes and thereby enabling application of SWNTs in various fields presents a very important technical challenge, and solving this challenge means making homogeneous chemical reaction possible.
Among the conventional technical means to disperse bundled SWNTs, non-covalent interaction and chemical functionalization are known. Examples of specific means to disperse bundled SWNTs are explained below.
The inventors proposed a method whereby SWNTs are sonicated in a mixed solution of sulfuric acid and nitric acid, and then sonicated again in a mixed solution of sulfuric acid and hydrogen peroxide, after which the SWNTs are chemically reacted with an amine in the presence of an organic amine and dehydrating agent to functionalize the SWNTs with the amine and thereby produce SWNTs that can be dissolved in solvents (refer to Patent Literature 1).
It is also known that processing carbon nanotubes containing fluorinated SWNTs in a solvent such as dimethylformamide would change the six-member ring structure and electron state of carbon atoms and consequently improve the hydrogen absorption capability of the material to a significant degree (refer to Patent Literature 2).
Also, a technique is known whereby SWNTs are uniformly dispersed in a solution containing organic polymer material to produce a coating material of high uniformity and this coating material is used to form on the surface of a substrate a uniform layer of a substance that contains SWNTs (refer to Patent Literature 3).
Another method is known whereby carbon nanotubes are dispersed easily and stably, without chemical functionalization, in a dispersion that contains a dispersing agent constituted by a chemical compound having a hydrophobic-hydrophilic structure (refer to Patent Literature 4).
Furthermore, the property of SWNTs dispersing at high content in an organic solvent in the presence of an amine is also known (refer to Non-patent Literature 2).
Although partially overlapping with the examples of prior arts described above, a method has been proposed whereby non-covalent bonds of polymers or 1-conjugated compounds and SWNTs are formed to disperse bundled SWNTs in a non-aqueous solution, without changing the structure or the structures and properties of bond constituents. However, spectroscopic characteristics of such SWNTs dispersed in a non-aqueous solution have not been reported.
In view of the situation, the inventors have proposed, after the application for the basic patent pertaining to the present application for patent, an invention regarding a processing/treatment method wherein bundled SWNTs comprising multiple SWNTs bonded together are added to a tetrahydrofuran solution containing octylamine, after which the solution is given vibration to separate the SWNTs into individual nanotubes to measure their absorption spectra using a spectrophotometer and thereby objectively confirm the degree of separation of bundled SWNTs into individual nanotubes (Japanese Patent Application No. 2004-310231).
In addition, other technical means are known for separating metallic carbon nanotubes and semiconducting carbon nanotubes, such as dielectrophoresis, chromatography, non-covalent interaction, and chemical functionalization. Specific examples of these means are explained below.
For example, a known literature is available that talks about a method to obtain semiconducting carbon nanotubes by utilizing the strong interaction between amines and semiconducting carbon nanotubes, wherein specifically an amine is added to SWNTs to separate semiconducting SWNTs (refer to Non-patent Literature 1). Also, there is another application for patent that was filed in the U.S. prior to the basic application for patent pertaining to the present application for patent (refer to Patent Literature 5). Here, the method described in Non-patent Literature 1 assumes that oxidization is given as a pretreatment, while the invention described in Patent Literature 5 assumes that oxidization or oxidization plus heating to high temperature is given as a pretreatment.
Furthermore, a method for refining carbon nanotubes is known wherein carbon nanotubes placed in a rotating drum are irradiated with electron beam to charge the nanotubes with electricity, and then metallic carbon nanotubes that could not be electrically charged are removed from the rotating drum to separate metallic carbon nanotubes from insulating carbon nanotubes (refer to Patent Literature 6).
In addition, there is a known method to separate semiconducting SWNTs by applying electrical current to SWNTs to selectively burn off metallic SWNTs, thereby removing metallic SWNTs and allowing only semiconducting SWNTs to remain (refer to Non-patent Literature 3).
Another known method of carbon nanotube separation is to conduct dielectrophoresis of SWNTs that have been dispersed with a surfactant, in order to separate metallic SWNTs and semiconducting SWNTs (refer to Non-patent Literature 4). This technique produces 1 pg of metallic SWNTs from 100 ng of a dispersion containing SWNTs (the effective yield is one one-hundred-thousandth of material).
Yet another known separation method involves anion exchange chromatography of a DNA dispersion of SWNTs, in order to separate SWNTs based on different diameters and electrical properties. In spectral analysis, SWNTs with smaller diameters as well as metallic SWNTs flow out more quickly, while SWNTs with larger diameters as well as semiconducting SWNTs flow out more slowly (refer to Non-patent Literatures 5 and 6).
Also, a method is known whereby semiconducting SWNTs are concentrated in a dispersion to a noticeable degree by utilizing the selectivity of a porphyrin derivative with respect to semiconducting SWNTs, which is considered a non-covalent interaction, and thus metallic SWNTs are concentrated preferentially into residue (refer to Non-patent Literature 7).
Furthermore, their electrical properties allow SWNTs to be chemically functionalized with a diazonium reagent with high selectivity, where metallic nanotubes react with diazonium, while semiconducting SWNTs are removed (refer to Non-patent Literature 8).    Patent Literature 1: Japanese Patent Laid-open No. 2004-168570    Patent Literature 2: Japanese Patent Laid-open No. 2004-313906    Patent Literature 3: Japanese Patent Laid-open No. 2001-011344    Patent Literature 4: Japanese Patent Laid-open No. 2003-238126    Patent Literature 5: U.S. Patent Laid-open No. 2004/0232073    Patent Literature 6: Japanese Patent Laid-open No. Hei 6-228824    Non-patent Literature 1: Debjit Chattopadhyay, Izabela Galeska, Fotios Papadimitrakopoulos, “A Route for Bulk Separation of Semiconducting from Metallic Single-Wall Carbon Nanotubes,” JACS Articles, J. Am. Chem. Soc., 2003, 125, 11, The Nanomaterials Optoelectronics Laboratory, Department of Chemistry, Polymer Program, Institute of Materials Science, University of Connecticut, pp. 3370-3375, Feb. 22, 2003    Non-patent Literature 2: Yuya Hirashima, Shin-ichi Kimura, Yutaka Maeda, Tadashi Hasegawa, Takatsugu Wakahara, Takeshi Akasaka, Tetsuo Shimizu, Hiroshi Tokumoto, “Interaction of SWNTs and Amines,” Proc. of the National Meeting of the Chemical Society of Japan, the Chemical Society of Japan, Mar. 11, 2004, p. 59    Non-patent Literature 3: P. G. Collins, M. S. Arnold, P. Avouris, Science, 2001, 292, Apr. 27, 2001, pp. 706-709    Non-patent Literature 4: R. Krupke, F. Hennrich, H. V. Lohneysen, M. M. Kappes, Science, 2003, 301, Jul. 18, 2003, pp. 344-347    Non-patent Literature 5: M. Zheng, A. Jagota, E. D. Semke, B. Diner, R. Mclean, S. R. Lustig, R. E. Richardson, N. G. Tassi, Nature Mater., 2003, 2, May 1, 2003, pp. 338-342    Non-patent Literature 6: M. Zheng, A. Jagota, M. S. Strano, A. P. Santos, P. Barone, S. G. Chou, B. A. Diner, M. S. Dresselhaus, R. S. Mclean, G. B. Onoa, G. G. Samsonidze, E. D. Semke, M. Usrey, D. J. Walla, Science, 2003, 302, Sep. 28, 2003, pp. 1545-1548    Non-patent Literature 7: H. Li, B. Zhou, Y. Lin, L. Gu, W. Wang, K. A. S. Fernando, S. Kumar, L. F. Allard, Y. P. Sun, J. Am. Chem. Soc., 2004, 126, Jan. 8, 2004, pp. 1014-1015    Non-patent Literature 8: M. S. Strano, C. A. Dyke, M. L. Usrey, P. W. Barone, M. J. Allen, H. Shan, C. Kittrell, R. H. Hauge, J. M. Tour, R. E. Smalley, Science, 2003, 301, Sep. 12, 2003, pp. 1519-1522